The present invention relates to a method for continuous or semi-continuous casting of metal or metal alloys into an elongated strand. The strand is cast using a device comprising a cooled continuous casting mold and an inductive coil arranged at the top end of the mold. The coil being supplied with a high frequency alternating current from a power supply. The invented method ensures that temperature and other casting conditions determining the initial solidification conditions in the mold are controlled such that a cast product, a strand, exhibiting an improved surface characteristics, a controlled cast structure, a low level of entrapped inclusions and other defects is produced at maintained or increased productivity. The present invention also relates to a device including the continuous casting mold, the coil, a power supply unit with control means suitable for the invented method.
During continuous or semi-continuous casting of metals and metal alloys, a hot metal melt is supplied to a cooled continuous casting mold, i.e. a mold which is open in both ends in the casting direction. The mold is preferably water-cooled and typically surrounded and supported by a structure of support beams. Melt is supplied to the mold where the metal is solidified and a cast strand is formed as it is passed through the mold. A cast strand leaving the mold, comprises a solidified, self-supporting surface layer or shell around a residual melt. Generally it can be said that conditions of initial solidification is critical for both quality and productivity. The conditions of initial solidification is dependent on a number of factors influencing each other in a complex manner, such as;
Metal flow in the upper part of the mold;
Lubrication between the mold and the melt/cast strand;
Heat losses and overall thermal conditions at the meniscus;
Thermal conditions and heat dissipation at the front of solidification; and
Oscillation, if any, of the mold.
A lubricant is typically supplied to the upper surface of the melt in the mold. The lubricant serves many purposes, amongst others it will prevent the skin of the cast strand first developed from sticking to the mold wall. Should the solidified skin stick or adhere more severely to the mold it will show as surface defects and in some cases as ripping of the first solidified skin. For large dimension strands of steel the lubricant is predominantly a so-called mold powder comprising glass or glass forming compounds that is melted by the heat at the meniscus. The mold powder is often continuously added to the upper surface of the melt in the mold during casting, as an essentially solid, free flowing particulate powder. The composition of a mold powder is customized. Thereby the powder will melt at a desired rate and lubrication will be provided at the desired rate to ensure lubrication. A too thick layer of lubricant between mold and cast strand will also affect the solidification conditions and surface quality in an undesired way, thus the thermal conditions at the meniscus need to be controlled. For smaller strands and for non-ferrous metals oil, typically vegetable oil, or grease is used as lubricant. Irrespective of what type of mold lubricant is used it should preferably be fed into the interface cast strand/mold at an even rate sufficient to form a thin uniform film in the interface to avoid surface defects originating from adherence between mold and strand. A too thick film might cause uneven surface and disturbs the thermal situation.
Heat losses and overall thermal conditions at the meniscus are predominantly controlled by the secondary flow that is developed in the mold. The use of inductive HF heaters for influencing the thermal situation at the top end is discussed in e.g. U.S. Pat. No. 5,375,648 and in earlier not yet published Swedish Patent Application No. SE9703892-1. High thermal losses are compensated by a supply of heat to the upper surface, either by a controlled upward flow of hot melt or by a heater, otherwise the meniscus can start to solidify. Such a solidification will severely disturb the casting process and destroy the quality of the cast product in most aspects.
A high frequency inductive heater arranged at the top end of a continuous casting mold will provide means to improve the capability to control the temperature of the metal at the upper surface of the melt, the meniscus, and the same time generate compressive forces acting to separate the melt and the mold, thereby reducing the risk for sticking, reducing oscillation marks and in general provide improved conditions for mold lubrication. The improved lubrication is primarily attributed to the compressive forces acting to separate the melt from the mold. The inductive heater may be of single-phase or poly-phase design. Preferably a high-frequency magnetic alternating field is applied. The compressive forces, generated by the high frequency magnetic field, reduce the pressure between the mold wall and the melt, whereby the conditions for lubrication are significantly improved. Surface quality of the cast strand is improved and the casting speed can be increased without risking the surface quality. Oscillation is preferably applied to ensure that the cast strand leaves the mold. However minor surface defects, so-called oscillation marks are normally formed on the cast strand upon contacts between mold and strand during the formation of the skin. These oscillation marks also effect the structure of the cast strand as inclusions often are trapped at them. As the compressive forces act to separate the melt from the mold they will minimize any contact between the melt and mold during initial solidification of the skin and improve the feed of lubricant thereby further improving the surface quality of the cast strand. Thus the use of multi-turn coil supplied with a high frequency alternating current and arranged at the meniscus is believed to provide a means to eliminate or at least substantially reduce the oscillation marks and thereby improving surface quality, internal structure, cleanliness and also productivity.
It is an object of the invention to provide a method for continuous casting of metal strand, wherein the conditions for the initial solidification of the cast metal in the mold are improved and in particular the conditions for mold lubrication is improved. In particular it is an object of the present invention to control the high frequency magnetic field, which is applied to act on the melt at the top end of the mold, such that the generated compressive forces acting to separate the melt from the mold ensuring a stable feed of mold lubricant into the interface between the mold and the strand and a formation of a lubricating film in the interface. Thereby can surface defects such as internal oscillation marks and any defects or productivity concern associated with them be essentially eliminated or at least substantially reduced. This is accomplished by the present invention, which according to one aspect provides a method for continuous or semi-continuous casting of metal according to the preamble of claim 1, which is characterized by the features of the characterizing part of claim 1. Further developments of the method are characterized by the features of additional claims 2 to 13.
It is further an object of the present invention to provide a continuous casting device comprising a cooled continuous casting mold, oscillation means, a multi-turn inductive coil supplied with a high frequency alternating current and a power supply unit with current control means to generate and control the high frequency magnetic field applied to act on the melt at the top end of the mold.
In particular the casting device shall comprise means to control the alternating current supplied to the high frequency magnetic field generating device such that casting conditions and operating parameters are optimized to accomplish quality improvements and/or productivity improvements. In particular shall the casting device be arranged with means such that forces acting on the melt and movements or flows in the melt are controlled such that the oscillation marks can be essentially eliminated or at least substantially reduced. It is further an object to provide a continuous casting device that ensures good and controlled thermal, flow, lubrication and overall conditions at the top end of the mold, thus attaining considerable improvements with respect to quality and productivity. This is achieved with a device for continuous casting of metals according to another aspect of the present invention, that provides a device for continuous or semi-continuous casting of metal according to the preamble of claim 14, which is characterized by the features of the characterizing part of claim 14. Further developments of the device are characterized by the features of additional claims 15 to 22.
The elimination or substantial reduction of oscillation marks further improves the cast structure and removal of inclusions as inclusions and/or defects are trapped at the oscillation marks.
A method for continuous or semi-continuous casting of metal wherein,
hot melt is supplied to a cooled continuous casting mold,
the melt is cooled and formed to a at least partly solidified strand as it passes through the mold, and
a high frequency magnetic field having a base frequency of from 50 Hz or more is applied to act on the melt at the top end of the mold using an inductive coil such that heat is developed in the melt and compressive forces acting to separate the melt from the mold wall are generated, whereby a current is supplied from a power supply to the coil for generation of the magnetic high frequency field, is according to the present invention carried out in a manner wherein the supplied current is controlled in a pulsed, amplitude modulated manner with an amplitude modulated modulation frequency of 10 Hz or less, whereby essentially full amplitude of the amplitude modulated current is achieved within a rise time corresponding to 1 cycle of the base frequency or less at the start of a pulse. This minimized rise time of the current amplitude at the start of each pulse cycle to full amplitude is essential to achieve the desired control of the compressive forces acting to reduce to the pressure between the mold wall and the melt at the top end of the mold. Hereby the conditions for lubrication are significantly improved and further they can be controlled by the amplitude modulated current supply. This offers a capability for improvements of surface quality of the cast strand and also for an increased casting speed without risking the surface quality. Preferably the rise time is minimized such that essentially full amplitude of the amplitude modulated current is achieved within a rise time corresponding to xc2xc (0.25) cycle of the base frequency or less at the start of a pulse.
It is when carrying out the method according the present invention preferred to supply a high frequency current with a base frequency of from 50 to 1000 Hz and to control this current supplied from the power supply to the inductive coil in a pulsed, amplitude modulated manner with an amplitude modulated modulation frequency of from 0.1 to 10 Hz. Preferably a high frequency current with a base frequency of about 200 Hz is supplied. The duty cycle of the high frequency current can be varied from 0 to 100% of the modulation frequency period.
According to one preferred embodiment the pulsed current is supplied in an essentially rectangular manner wherein the output current is varied between two levels. The output current can then be supplied in an on-off manner, wherein in the output current in the off-periods is an essentially zero output current. Alternatively the pulsed current is supplied in an essentially rectangular manner between two current amplitude levels, wherein the output current at both levels is separated from zero.
According to one further embodiment also the time-period for fall of the current amplitude at the end of a pulse is minimized to a time corresponding to 1 cycle of the base frequency of the high frequency current or less, preferably to a time corresponding to xc2xc (0.25) cycle of the base frequency of the high frequency current or less.
Typically the mold is oscillated during casting and when the method according to the present invention is carried out in such an oscillated mold it is often favorable to adopt a preferred embodiment of the present method in which the pulsed modulation frequency of the amplitude modulated current is associated with the frequency of the mold oscillation, such that the variations in the compressive forces are coordinated with the mold oscillation.
The current is then according to a preferred manner pulsed with a modulation frequency in the same order as the oscillation frequency but the pulsed frequency and the oscillation frequency can also be associated in any suitable manner which generates a control of the compressive forces acting to reduce to the pressure between the mold wall and the melt at the top end of the mold.
A suitable device for carrying out a method for continuous or semi-continuous casting of metal according to the present invention comprises,
a cooled continuous casting mold,
means for supplying hot melt to the mold,
means for extracting and/or receiving a cast strand formed in the mold from the mold,
an inductive coil arranged at the top end of the mold to, when supplied with an alternating electric high frequency current, generate a high frequency magnetic field to act upon the melt in the mold, whereby heat is developed in the melt and compressive forces acting to separate the melt from the mold wall are generated, and
a power supply unit with current control means to supply an alternating electric high frequency current having a base frequency of 50 Hz or more to the inductive coil, wherein the current control means according to the present invention further comprises modulation means for modulation and control of the supplied current in a pulsed, amplitude modulated manner with a modulation frequency of 10 Hz or less, whereby essentially full amplitude of the amplitude modulated current is achieved within a rise time at the start of a pulse corresponding to 1 cycle of the base frequency or less.
The current control means comprises means, which dependent on the continuous casting machines and casting variables modulation is adapted;
for on-off modulation of the load coil current supplied to the coil;
for modulation between any two current levels of the load coil current supplied to the coil;
for modulation of waveform envelope shapes having any periodic waveform pattern such as sine, triangular, or trapezoidal modulation envelopes of the load coil current supplied to the coil; or
for a programmable arbitrary modulation waveform pattern generation of the load coil current supplied to the coil.
According to one embodiment the current control means comprises a converter with a series resonant circuit with modulation means for supplying a current with an amplitude modulation pattern exhibiting an essentially rectangular wave configuration. Preferably the modulation means arranged for supplying a current with an amplitude modulation pattern exhibiting an essentially rectangular wave configuration with off periods alternating with on periods in a supply frequency, wherein the off and on periods comprise a plurality of cycles of the base frequency of the amplitude modulated current supplied to the inductive coil.
The series resonant circuit used in one preferred embodiment typically comprises a quench thyristor in parallel with a DC smoothing reactor, that the thyristor, in the on-off rectangular modulation mode is adopted to;
control the voltage to which the series resonant capacitor is charged at the end of each on modulation period to an optimal level, and
trap the energy stored in the smoothing reactor during the off period with in each modulation cycle, thereby a release enabling of the energy into an inverter circuit at the beginning of the next on modulation period. These two functions being critical to the attainment of a nearly perfectly rectangular leading edge modulation envelope shape of the output current of the converter, which in turn is necessary for optimal surface and metallographic microstructure of the continuously cast end product.
According to one alternative embodiment the modulation means are arranged for supplying a current with a modulation pattern exhibiting an essentially rectangular wave configuration varying between two levels, that the current amplitude is held essentially constant at these two levels for time periods comprising a plurality of complete cycles of the base frequency of the amplitude modulated current supplied to the inductive coil.